Study of fundamental physical properties of some ternary nitride-based compounds (Ca5Si2N6 and Sr5Ge2N6)

Abstract

Our work consists in studying the structural, elastic, electronic, optical, thermoelectric and thermodynamic properties of the ternary nitrides Ca5Si2N6 and Sr5Ge2N6, using the density functional theory (DFT) within the GGA-PBEsol and TB-mBJ approaches. The theoretically predicted lattice parameters constants are consistent with the available experimental results. According to the estimated cohesive energy, formation enthalpy and Born-Huang stability criterion, the examined compounds are thermodynamically and mechanically stable. Findings on elastic properties suggest that both nitrides are ductile in nature with significant elastic anisotropy. Using the TB-mBJ electronic band structure, these nitrides were found to be semiconductors with a direct band gap (V-V) of 3.55 eV for Ca5Si2N6 and indirect band gap (V-Γ) of 3.15eV for Sr5Ge2N6. The nitridosilicate compound can be used as a phosphor material because the band gap and Deby temperature values are within the recommended range. Substitution with heavier cations leads to narrowing the Eg value and reducing the ϴD(K) value. Even that, this material is still useful for other optical applications. The computed optical functions show a modest static dielectric, a notable optical anisotropy and a strong absorptivity of electromagnetic waves in the ultraviolet range, indicating that the title compounds are suitable for UV-optoelectronic applications. Furthermore, transport properties, including Seebeck coefficient, electrical conductivity, thermal conductivity, power factor and figure of merit have been studied employing classical BoltzTrap software. The large figure of merit (ZT) values of these ternary nitrides, suggests their potential use in thermoelectric devices, especially at low and room temperature. Using Debye's quasi-harmonic model, we explored the thermodynamic properties of Ca5Si2N6 and Sr5Ge2N6 under the effect of temperature and pressure such as: the modulus of compressibility B, coefficient of thermal expansion α, heat capacity at constant volume CV, heat capacity at constant pressure CP and the Debye temperature ϴD(K) .